RFR: 8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code [v5]

[Archie L. Cobbs]

> This bug and a few others fall into the "output file clash" bucket. This is when the compiler thinks it's writing out two separate files, but due to the way the O/S filesystem maps `Path`'s to actual files, it's really writing the same file twice.
> 
> This is usually due to case-insensitive filesystems, but can also be due to how a filesystem "normalizes" file names. For example, on MacOS, compiling this class will generate such a clash:
> 
> public class Test {
>     interface Cafe\u0301 {
>     }
>     interface Caf\u00e9 {
>     }
> }
> 
> The reason is that `\u0301` is the Unicode character "Combining Acute Accent" which means "stick an accent over the previous character". So MacOS normalizes a `e` followed by a `\u0301` into a Unicode `\u00e9`, that is, `é`. However, the Java language treats these the two names `Cafe\u0301` and `Caf\u00e9` as distinct.
> 
> It's infeasible to truly "fix" this problem, so we resort here to a salve, which is to add a new compiler flag `--detect-output-file-clashes` that enables detection of output file clashes. When the flag is enabled, and a clash is detected, an error is immediately thrown. For example, compiling the example above gives this:
> 
> $ javac --help-extra
>   ...
>   --detect-output-file-clashes
>         Generate an error if any output file is overwritten during compilation. This can occur, for example,
>         on case-insensitive filesystems. This applies to class files, native header files, and source files.
>   ...
> $ javac --detect-output-file-clashes Test.java
> Test.java:4: error: error while writing Café: output file clash: /Users/archie/test/Test$Café.class
>     interface Caf\u00e9 {
>     ^
> 1 error
> 
> This at least gives people at risk of encountering this problem a way to turn a runtime error into a compile-time error.
> 
> **Outstanding Questions**
> * Is making this optional via an (extended) flag `--detect-output-file-clashes` the best way to address this?
> * Does the new field `BaseFileManager.outputFilesWritten` ever need to be cleared? Can the file manager be used for multiple compilations? If so, is there some "reset" step we should hook into?

Archie L. Cobbs has updated the pull request incrementally with one additional commit since the last revision:

  Convert "output-file-clash" from an error into a warning.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12754/files
  - new: https://git.openjdk.org/jdk/pull/12754/files/b255ed85..7d2af149

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12754&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12754&range=03-04

  Stats: 11 lines in 3 files changed: 6 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/12754.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12754/head:pull/12754

PR: https://git.openjdk.org/jdk/pull/12754